The initiation and propagation of coating morphological and structural defects under mechanical strain and their effects on the electrochemical behaviour of pipeline coatings

摘要

Polymeric coatings on buried high pressure steel pipelines are subjected to multiple complex influences of mechanical stresses, the underground soil corrosion environment and cathodic protection (CP). It is known that an applied mechanical strain can cause the formation of morphological and structural defects in coatings; however the impact of such mechanically induced defects on the coating's corrosion resistance has not been examined sufficiently. This paper reports an investigation of the influence of such coating defects on a typical high build epoxy pipeline coating by assessing the coating's electrochemical properties using electrochemical impedance spectroscopy (EIS) and a new tensile electrochemical testing setup. In situ and ex situ scanning electron microscopy was employed to trace and characterise coating defects from the very early stages of micro shear bands initiation to the formation of full coating cracks. Finite element analysis has been employed to understand the influence of strain on the initiation and propagation of coating defects by modelling stress distributions. A correlation has been found between the applied strain levels and the corresponding coating resistance and capacitance values. The strain distribution and shear stress distribution patterns obtained using the finite element analysis were used to explain typical features observable in EIS data based on the formation of electrolytic pathways through unstrained and strained coatings. This approach has been consolidated into a two dimensional model proposed to explain the electrolyte movement in a coating impacted by applied mechanical strain and environmental exposure.